An apparatus that measures a three-dimensional shape of an object is generally called a three-dimensional scanner, and apparatuses based on various principles are known. For example, Japanese Unexamined Patent Publication No. 2001-12942 discloses a three-dimensional scanner that carries out shape measurement by detecting a state of contact between an object to be measured and a contact probe by a sensor and measuring three-dimensional positions of the object surface. Further, Japanese Unexamined Patent Publication No. H07-91930 discloses a three-dimensional scanner that carries out shape measurement by irradiating a light beam onto the respective portions of an object from a light source and measuring distances from the light source to the respective portions based on intensities of reflected lights.
On the other hand, a method for obtaining a three-dimensional shape by means of an arithmetic operation by analyzing two-dimensional images of an object imaged with CCD cameras, etc., has also been proposed. For example, Japanese Unexamined Patent Publication No. H07-91927 discloses an apparatus for measuring a three-dimensional shape by means of analysis using stereography by imaging a state of projection of pattern light onto an object from different directions with two cameras. In particular, it is generally called a light section method to measure a three-dimensional shape of an object by imaging a state of projection of a line pattern of light onto the object and analyzing obtained two-dimensional images.
Recently, the image analysis technology using a computer has been improved, so that the apparatus for shape measurement using the light section method has also been put into practical use as an apparatus to be mass-produced on a commercial basis, and is variously devised. For example, Japanese Unexamined Patent Publication No. 2001-255125 discloses a three-dimensional shape measuring apparatus in which a slit projector is disposed around an object to project a line pattern of light transmitted through a slit, and Japanese Unexamined Patent Publication No. 2010-14505 discloses a three-dimensional shape measuring apparatus capable of improving measurement accuracy by devising the arrangement of a projector and an imaging device. Further, Japanese Unexamined Patent Publication No. 2003-50112 discloses an apparatus for carrying out three-dimensional shape measurement by projecting a line pattern consisting of stripes including a large number of lines onto an object and analyzing the stripes on the image of the line pattern.
When a three-dimensional shape is thus measured by projecting a line pattern consisting of one or a plurality of lines onto an object and analyzing an image of the line pattern, it is required to project a line pattern with accuracy as high as possible. Conventionally, a line pattern is formed by condensing light by using an optical system such as a lens, or a line pattern is formed by transmitting light from a light source through a slit, however, the accuracy of the formed line pattern is not always sufficient. For example, when a method for shaping by a slit is adopted, the structure is simple, but reflection and diffraction occur on the slit inner surfaces so that the contour of the line pattern easily becomes unclear.
Further, it is desirable that a line pattern to be formed on an object surface is a pattern with brightness as high as possible in order to extract the line pattern on an image. Therefore, a high brightness discharge lamp such as a high pressure mercury lamp must be used as a light source, however, such a special light source has a comparatively short service life, and must be frequently replaced.
An ideal light source that can solve this problem is a coherent light source such as a laser light source. For example, a semiconductor laser that has been widely industrially utilized is capable of carrying out illumination with high brightness while its service life is extremely long as compared with a high brightness discharge lamp such as a high pressure mercury lamp. The semiconductor laser is also excellent in directionality, so that even without shaping by an optical system or a slit, a line pattern having a clear contour can be projected.
However, illumination using a coherent light source such as a laser poses a new problem of occurrence of speckles. Speckles form a spot-like pattern that appears when coherent light such as laser light is irradiated onto a diffusing surface, and are observed as spot-like unevenness in brightness when laser light is projected onto an object surface. Therefore, when a line pattern is projected onto an object by using a laser light source, speckles occur on the line pattern on the object surface, and brightness unevenness occurs.
For example, when one point on a screen is indicated with a laser pointer, a spot of the laser light appears as a bright glare on the screen. This is caused by the occurrence of speckle noise on the screen. It is considered that the reason why speckles are caused when coherent light is used is that coherent lights reflected by portions of the diffusing and reflecting surface of a screen, etc., interfere with each other due to extremely high coherency. For example, a detailed theoretical consideration on occurrence of speckles is described in “Speckle Phenomena in Optics, Joseph W. Goodman, Roberts & Co., 2006.”
In use as a laser pointer or the like, only a small spot is seen by an observer, so that a severe problem does not occur. However, if a line pattern used for measuring a three-dimensional shape includes brightness unevenness based on such speckles, a contour shape of the line pattern on an image cannot be accurately extracted, and correct shape analysis cannot be carried out.
Of course, several detailed methods for reducing the above-described speckle noise have been proposed. For example, Japanese Unexamined Patent Publication No. H06-208089 discloses a technology for reducing speckles by irradiating laser light onto a scatter plate, utilizing scattered light obtained therefrom for illumination, and rotary driving the scatter plate by a motor. Further, Japanese Unexamined Patent Publication No. 2004-144936 discloses a technology for reducing speckles by irradiating laser light generated by a laser light source onto a scatter plate and utilizing the laser light as illumination, and oscillating this scatter plate. However, to rotate or oscillate the scatter plate, a large-scale mechanical drive mechanism is required, so that the entire apparatus is increased in size and also increased in power consumption.
Therefore, an object of the present invention is to provide a scanner apparatus and an apparatus for measuring a three-dimensional shape of an object, which have a function of projecting a line pattern onto an object by using a coherent light source, and can reduce occurrence of speckles on the surface of the object.